1. Field of the Invention
The present invention relates generally to the field of damped motion control such as suspension devices. More particularly, the present invention relates to a magnetorheological twin-tube damping device having tunable force characteristics controlled by a single magnetorheological valve component.
2. Description of the Related Art
In vehicle use, vibrations, shock and motion (resulting from uneven road surfaces, for example) are transferred to an operator through the vehicle""s structure. These conditions may lead to dangerous vehicle operation, and over time, to operator fatigue, discomfort and possibly reduced health. Typically, seats and cabs used in commercial and public transportation vehicles, such as trucks and buses, include suspension systems that aid in reducing the discomfort felt by the vehicle operator. These suspension systems typically include a mechanical device, such as a damper, attached between two structural members, such as a frame and a vehicle chassis. Dampers are well known which use a fluid as the working medium to create damping forces/torques and to control vibration, motion and shock. In particular, controllable dampers are well known that include electrorheological fluid (ER), electrophoretic fluid (EP), magnetorheological fluid (MR), hydraulic fluid, etc. In simple suspension systems passive dampers may be used. In more complicated suspension systems, adjustable dampers and controllers may be used to control the movement of the damper components and prevent end stop collisions.
Various methods have been employed to control vibration in suspension systems. Generally, in such prior art control methods, operating conditions are obtained by at least one sensor which supplies system operating information to a processor that determines the appropriate primary control signal to be sent to an electromechanical device, such as a magnetorheological (MR) fluid damper, for controlling vibration. A number of the various prior art methods for controlling vibration are described in the following issued United States patents: xe2x80x9cSkyhook Controlxe2x80x9d as described in U.S. Pat. No. 3,807,678 to Karnopp et al.; xe2x80x9cRelative Controlxe2x80x9d as described in U.S. Pat. No. 4,821,849 to Miller; xe2x80x9cObserver Controlxe2x80x9d as described in U.S. Pat. No. 4,881,172 to Miller; xe2x80x9cContinuously Variable Controlxe2x80x9d as described in U.S. Pat. No. 4,887,699 to Ivers et al.; xe2x80x9cDelayed Switching Controlxe2x80x9d as described in U.S. Pat. No. 4,936,425 to Boone et al.; xe2x80x9cDisplacement Controlxe2x80x9d as described in U.S. Pat. No. 5,276,623 to Wolfe; xe2x80x9cRate Controlxe2x80x9d as described in U.S. Pat. No. 5,652,704 to Catanzarite; xe2x80x9cModified Rate Controlxe2x80x9d as described in U.S. Pat. No. 5,712,783 to xe2x80x9cMethod for AutoCalibration of a Controllable Damper Suspension System as described in U.S. Pat. No. 5,964,455 to Catanzarite; and xe2x80x9cEnd Stop Control Methodxe2x80x9d as described in U.S. Pat. No. 6,049,746 to Southward et al.
A simple conventional controllable damper technique for suspension control involves using a controlled bleed arrangement to shape the force velocity curve in the low-speed region, and adjusts the control signal to the damper to control the desired level of control. One damper is used for each corner of the car, for example. However, there is no form of adjustable end stop control in these conventional systems.
A more advanced suspension control system used to maintain comfortable driving conditions by reducing vibration, shock, and motion is the Motion Master(copyright) Ride Management System offered for sale by the Lord Corporation (Lord Corporation, Cary, N.C.). The Motion Master(copyright) system is made up of four components. The first component is a position sensor operable for monitoring movement of a seat caused by changing road and operating conditions. The second is a controller that continuously receives signals from the position sensor, determines optimal damping forces and is capable of adjusting at a rate of about 180 times per second. The third is a shock absorber (damper) operable for responding to the controller within milliseconds. The last component is a ride mode switch that enables the vehicle operator to choose among different ride options, such as soft, medium and firm settings based on the operator""s preference. The position sensor and shock absorber are separate units that are mounted between two structural bodies. The shock absorber contains a magnetically responsive fluid that when subjected to a magnetic field is capable of changing from a near-solid to a liquid and back in a matter of milliseconds. The stronger the magnetic field, the more viscous the fluid and the greater the damping force. Such MR shock absorbers are typically of monotube construction in which the controllable fluid valve device is incorporated into the piston of the damper. Major disadvantages of such configurations include the need for very tight dimensional tolerances of the components that define the flow passages in the piston, and an associated high cost to manufacture.
What is needed is a simple and inherently low-cost damping device and control method that overcomes the disadvantages discussed above.
In a first aspect, the present invention describes a stationary, controllable fluid valve device comprising two bodies held in close proximity to one another that together form a complete magnetic circuit. A space between the two bodies defines a fluid pathway for magnetically responsive fluid. The two bodies are so designed that the magnetic flux in the gaps between the bodies is perpendicular to the fluid flow. The first annular body incorporates all the features that define the necessary gaps and passageways for magnetorheological fluid flow control. The second body incorporates magnetic field generating means such as an electric coil. A magnetic flux is generated by the magnetic field generating means and is carried by the magnetic circuit. The magnetic flux is directed to act upon the magnetorheological fluid in controllable passageways to provide controllable rheology changes, thereby restricting flow through the controllable passageways.
In a second aspect, the present invention describes a twin-tube damping device including the controllable fluid valve device disposed in a stationary position within the damping device. The twin-tube damping device is further comprised of an outer member containing an outer volume of magnetorheological fluid, an inner member containing an inner volume of magnetorheological fluid, a piston rod mounted for movement with respect to the inner and outer members and a piston assembly attached to the piston rod. The piston assembly divides the inner volume of magnetorheological fluid into a first fluid-containing chamber and a second fluid-containing chamber. The controllable fluid valve device is operable for permitting magnetorheological fluid to flow from the first fluid-containing chamber to the second fluid-containing chamber, and from the second fluid-containing chamber to the first fluid-containing chamber. The damper also includes a pressurized compliance means to accommodate the fluid volumetric changes caused by both rod displacement and temperature effects.
In a third aspect, the piston assembly may comprise a pressurized accumulator. In the embodiment employing a pressurized accumulator, the piston rod may be a hollow rod that allows for pressurizing the damping device after assembly and magnetorheological fluid filling. The damper device may be filled via a hole in the center of a magnetic core that provides a convenient passage for evacuating and filling the damper device while limiting the reduction of flux area for the magnetic circuit.
The controllable fluid valve device of the present invention provides for the control of tunable force characteristics by a single magnetorheological valve component. Such a damper configuration may be controlled by a real-time control system, however, the nature of construction is such that it can provide the necessary features required by simplified controls such as basic adjustability and end stop control. The single fluid valve component is operable for the overall viscous damping level, magnetic damping strength and low-speed damping profile.